This figure is responsible for the slaying of the 
This figure is responsible for the slaying of the famed warrior Aeschere, whose head is found by this figure's lair. This figure lives beneath a lake, and the protagonist must swim for almost a day to reach the lake's bottom. The protagonist kills this creature using a giant sword he finds, as this monster is invulnerable to the blade Hrunting, which was given to the protagonist by Unferth immediately before the battle. Like its offspring, this creature is said to be a descendant of the first murderer, Cain. After killing this monster, the protagonist then decapitates the body of her monstrous son. For 10 points, identify this unnamed monster, killed by the slayer of her son, Beowulf.
This figure is responsible for the slaying of the famed warrior Aeschere, whose head is found by this figure's lair.
{Grendel's} Mother [accept equivalents]